Inductive heating coil



Aug. 7, 1,945. I. H. G. FROSTICK ETAL 2,381,274

" INDUCTIVEVHEATING COIL I Gnemibr Filqd Oct. 31, 1942 WITNESSES:

ATTbRNE'Y Patented Aug. 7, 1945 INDUCTIVE HEATING COIL Harold G. Frostick, Pittsburgh, and Elvyn M.

Sollie, Saxonburg, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh,

Pa., a corporation of Pennsylvania Application October 31, 1942, Serial No. 464,042

Claims.

Our invention relates to inductive heatingcoils, and particularly to such coils which are utilized to heat a rapidly moving, long, thin, fiat metal strip. or one or more wires, or other elongated member which is moving axially through the heating coil. In many inductive heatingcoils of this nature, where very high power must be put into the coil, the voltage of the coil may be a distinct limitation as to its design, and particularly the maximum voltage appearing between the coil-terminals and the metal strip or other work to be heated, within the coil.

For the most'eihcient operation, the diameter of the coil, or the cross-sectional area which is embraced within the inner periphery of the coil, should be as small as possible, particularly when the work to be heated is non-magnetic; and heretofore the coil has been either wound directly on an insulating liner or guide-member for the strip, wire, or other work to be heated, or when removable liners have been utilized, the space between the liner-and the inner periphery of the coil has been made as small as practicable. While this sort of construction is satisfactory for many lowpower and medium-power coils, it is not at all satisfactory for high-power work, as when the coil is to be supplied with several times as much dielectric material is disposed between the inner periphery of the coil and the. inside of the liner or guide-means which is liable to be contacted by the work to be heated, the major portion, or a not quite low. I

In accordance with our present invention, we.

avoid this difliculty by utilizing only a relatively thin insulating liner or separating-means, only thick enough to have the mechanical strength necessary for guiding the moving strip, wire, or

other work to be heated; and we make our inductive heating-coil larger than the liner,so that there is an air-gap ofiat least one-eighth to onequartenof an inch, or other suitable distance, usually more than a quarter of an inch, between the outer periphery of the liner and the inner pe-' riphery of the coil. With this construction, almost all of the drop in the electrostatic field-potential, between the coil and the strip or work, occurs in this airgap, because of its low dielectric constant power as the largest radio broadcasting station in the United States. When a very large amount of oscillation-frequency power is supplied to an inductive heating-coil, means must be provided for preventing corone or space-current discharges at the work to be heated. as a result of the electrostatic fleld-stress between the work and the highvoltage portions of-the coil. This difllculty has been met, heretofore, by shielding-means, with the a result that no small amount of diflicult y has been experienced with the shielding means which has been introduced for the purpose of eliminating coronairom the work to be heated. i In accordance with out present invention, we provide a design in which, in general, coronashielding means are avoided altogether, -or even 'if they are necessary, their design is very greatlyfacilitated by the reduction which we are enabled to'eflect in the electrostatic field-intensity in theof approximately unity, whereas only a relatively small potential-difference exists between the inner and outer surfaces of the liner, because of the higher dielectric constant of the liner. As a consequence of this construction, a high electrostatic field-stress does not exist between the inner surface of the liner and the outer surface of the strip or work, so that corona is, in general, prevented, or. if it still exists, the problem of shielding it is very considerably reduced by reason of the reduced electrostaticstresseswhich we obtain at our liner,

by using the air-gap spacing between the liner and the coil.

While our invention is not limited to any particular application, the special design of coil which we have introduced appears to find its most important present application in the tinplate-flowvicinity of the liner or guide-means which is disposed close to the work being heated, within the inductive heating-coil. "Our invention takes advantage oi. the fact that the specific inductive capacity or dielectric constant of any available solid insulator which can be utilized for the liner or separating means is several times higher than unity, that is, several times that of air or a gaseous medium. Hence, it a solid insulating or ing apparatus which is described and claimed in an application of Glenn E, Stoltz and Robert M. Baker, Serial No. 464,040, filed October 31, 1942,

and assigned to Westinghouse Electric Corpora- I tion.

In this application or our invention, 9. mileslong, thin, flat, electrolytically tinplated steel strip, having a gauge usually of the order of 0.008

to 0.011 inch, and having a width of from 28 to 36 inches, is passed. at a speed of the order of v eleven or twelve miles per hour through an inductive heating coil, which is required to heat the strip some 300 or 400 F. within as short a1 distance as possible, or at least within a distance which is within a reasonable factor of safety of the distance which would produce buckling of the strip as-a result of the strains th'erein, from thermal expansion, exceeding the elastic limit or a yield point of the strip. This high rate of heatinput into the strip is needed in order to make it possible to heat-treat the plated strip in tho same line or manufacturing process in which the strip is electroplated, dried, heated, quenched, cleaned, and otherwise treated.

The heating is for the purpose of raising the tinplate to or slightly above, its melting point, which is 452 F., so that the surface-tension of the molten tin will cause it to flow evenly over strip in the same fline" in which it had been electroplated, so that separate manufacturing operations will not be needed; and the use of induction heating is necessary in order to be able to get enough heat into the strip in a small space, to make the strip-heating substantially independent of slight variations in gauge, to avoid physical contact with the easily marred tin surface, and to provide the high-speed heatinput control which is necessary for satisfactory operation on these high-speed tinning lines.

Our present invention provides what appears to us to be the most satisfactory, or one of the most satisfactory, coil-designs which is capable of handling the high-power concentrations which are required of the work Just described.

Our invention consists in the apparatus, combinations, systems, parts and methods hereinafter described and claimed, and illustrated in the accompanying drawing wherein:

Figure 1 is a diagrammatic view of circuits and apparatus embodying our invention, with our inductive heating-coil shown in section, looking edgewise on the strip or work to be heated; and

Fig. 2 is a similar view form of coil-support.

We have illustrated our invention as applied to the inductive heating of a thin magnetizable metal strip l, which is shown edgewise in the drawing, and which is continuously moving downwardly, in the direction of its length, at the rate of 1000 feet per minute, more or less, within an inductive heating-coil 2 which comprises a plurality of turns of a hollow conductor, wound in a single layer, through which coolingwater or other coolant is circulated, as by means of a grounded water-inlet 3 at the midpoint of the coil, and insulated water-outlets 4 and 5 at the coil-terminals! and 7, respectively, which are supplied with a heavy high-frequency current from an oscillation-generator 8.

A solid insulating separating-means I0 is disposed within the coil 2, with portions of said separating-means on opposite sides of the strip 1 or other work to be heated. The solid insulatingseparating-means lb is preferably in the showing a modified form of a flattened tubular liner of asbestos or a molded composition-product having woven glass reinforcement layers embedded therein to provided for avoiding excessive friction with the strip, 01, if practicable, to avoid any contact with the strip in normal operation, although an occasional light contact is frequently permissible.

It is an essential feature of our invention that a space shall be provided between the inner periphery I! of the coil and the outer periphery l3 of the liner In, this space being occupied by substantially nothing but air or other gaseous ambient medium, particularly at the ungrounded or high-voltage portions of the coil, so as to provide an air-gap of at least one-eighth to onequarter of an inch, or such other spacing, usually more, as may be necessary in any particular ap- Dlication.

Any suitable means is provided for supporting both the insulating liner in and the coil 3 with a suillcient amount of rigidity. In the form of our invention shown in Fig. 1, an outer insulating housing I6 is provided, in spaced relation around the coil 2, and the outer peripheries of the coil are supported, at a plurality of points, by pin-type insulators l1, extending radially between the coil and the outer insulating housing or supporting-means I6.

Preferably, both the liner I0 and the outer housing It extend beyond the ends of the coil 2; and annular end-closure means I8 and I9, of suitable insulating material, are provided for Joining the liner l0 and the housing-member I! at each 'of the respective ends, and to cooperate with the housing-member and the liner to provide a complete enclosure for the coil 2, while at the same time supporting the ends of the liner in. In many cases, it is quite desirable that the enclosure-means for the coil shall be dustproof, so as to shield thev coil from accumulations or condensations of tin-dust, or steam, or any other foreign matter which may tend to cause a breakdown of the coil. At the same time, the annular end-closure means. l8 and [9, provide excellent supporting-means for the liner l0, disposed a considerable distance beyond the axial extent of the coil 2, where the alternating magnetic field of the coil is relatively weak.

In the form of our invention which is shown in Fig. 2, instead of utilizing the outer housingmember 16 and the pin-type insulators I! extending inwardly therefrom, we embed the outer periphery of our coil 2' in a solid insulating member 20 of asphalt, casting resin, or other suitable molded insulating material, which rigidly holds the coil in place, while leaving the inner periphery i2 of the coil-turns substantially free of any material amount of solid insulation. In other respects, the design shown in Fig. 2 is similar to that shown in Fig. 1;

In operation, as previously indicated, the airgap which is provided between the liner l0 and the coil 2 or 2 provides a space in which most of the electrostatic stress between the coil and the strip or work i is consumed, thus materially reducing the electrostatic field-concentration in the vicinity of the liner in, usually reducing this electrostatic field to' the point where corona is not obtained on the strip or work I. If corona should be obtained in any case, this phenomenon can be'avoide'd by redesigning the coil so as to have a larger air-gap between the coil and the liner;

to heated, due .to a high electrostatic work - stress, is an important consideration in the deand the coil, without excessively reducing the heating-efllciency or the proportion of the coilflux which threads the work, so as to produce eddy-currents therein. v

While we have illustrated our invention in two diflerent forms of embodiment, which are at present preferred by us, we desire it to be understood that our invention is not limited to these particular forms. We desire, therefore, that our,

claims shall be accorded the broadest construction consistent with their language.

We claim as our invention: I

I 1 An inductive heating apparatus operable in' a gaseous medium at approximately atmospheric pressure and comprising an induction heatingcoil comprising a plurality of turns of a flowingfluid-cooled conductor, wound in a single layer, a coil-supporting means disposed outside o! the sign.

2'. 'I'he'invention as defined in claim 1, characterized by said coil-supporting means comprising a housing-member surrounding the coil in spaced relation around the outer periphery of the coil, and a plurality of relatively long, spaced insulators connected at a plurality of points to the outer peripheries of a plurality of the turns of the coil for supporting the outer periphery of the 0011 from said housing-member.

3. The invention as defined in claim 1, 'characterized by said coil-supporting means comprising a housing-member surrounding the coil in spaced relation around the outer periphery of the coil, and a plurality of relatively long, spaced insulators connected at a plurality of points to the outer peripheries of a plurality of the turns of the coil for supporting the outer periphery of the coil from said housing-member, said housouter periphery of the coil and extending outwardly from the outer periphery of the coil ior supporting the coil, the portions or the coil-supporting means which are disposed closest to the attachment-point to the coil, and for a considerable radial distance out therefrom, being ,substantially altogether of insulating material, a solid insulating liner disposed within the coil and having an axially extending opening therein for accommodating the work to be heated, the outer periphery of the liner being spaced from the inner periphery of the coil and bei'ngseparated from at least the high-voltage portions of the coil essentially by a gaseous ambient medium, said liner having extended portionsv thereof extending axially beyond the axial length of the coil at each end, an insulating liner-supporting means, spaced axially beyond ,each end of the coil, for engaging said extended portions or the liner, and means for supplying such a large highfrequency current to the coil that the danger or space'current discharges from the coil to the ing-member and said liner both having an axial extent greater than that of t and said liner-supporting means compr ing en means joining said liner and said housin her at each of the respective ends the of to cooperate with the housing-member and to provide a complete enclosure for the 4. Thednvention as defined in claim acterized by said cod-supporting m ing a solid mass of insulating material in which the outer portions of the coil-turns are embedded, leaving the inner periphery of the coil-turns substantially free or any material amount of solid insulation.

5. The invention as defined in claim 1, characterized by said coil-supporting means comprising a solid mass of insulating materialin which the outer portions of the coil-turns are embedded,

leaving the inner periphery of the coil-turns substantially tree of any material amount of solid insulation, and aid liner-supporting means comprisingend closure means Joining said linerto said solid mass of insulating material to provide a complete enclosure for the coil.

' HAROLD G, Faos'rIcK. A ELVYN M. scum 

